Improve awareness of theme contributing guide (THEME_CONTRIBUTING.md)

[neiljp]

This document is not linked from anywhere; this could likely go somewhere like the FAQ, maybe elsewhere, and perhaps even in the developers/contributing guide?

This was originally raised by @Rohitth007 in #1208. The table in that document - or document fragment, if we later use it elsewhere - seems useful to keep concise, but we could add paragraphs describing some elements in more detail further down perhaps.